Detect already-undone edits for undo

Detect already-undone edits for undo

When an edit has already been undone by a later edit, the current
behavior of displaying undo-success with an empty diff tends to confuse
users. It seems it would be better to detect this situation and display
a message along the lines of "This edit has already been undone".

Bug: 29762
Change-Id: Ia40a06046a72b0de52a2b75165ca105bbf4e2e6a